[JS] Data Types - Iterables

1. String is Iterable

String can use for..of loops to iterate over its characters.

for (let char of 'text'){
  console.log(char); // t, then 2, then x, then t
}

2. Iterables and Array-Likes

Iterables and Array-Likes are different from each other.

• Iterables are objects that implement the Symbol.iterate (for..of method) method.
• Array-Likes are objects that have indexes and length.

String would be both an iterable and array-like, because for..of and length works. However, an object would be an array-like, not iterable.

let arrayLike = { // has indexes and length => array-like
  0: "Hello",
  1: "World",
  length: 2
};

// Error (no Symbol.iterator)
for (let item of arrayLike) {}

3. Array.from

Then, how can we make an iterable or array-like value to a real Array? We can use Array.from for this.

let arrayLike = {
  0: "Hello",
  1: "World",
  length: 2
};

let arr = Array.from(arrayLike); // (*)
alert(arr.pop()); // World (method works)

Also, we can add a mapping function to it.

// square each number
let arr = Array.from(range, num => num * num);

alert(arr); // 1,4,9,16,25